This invention relates to an active matrix display device using two-terminal non-linear switching devices, and in particular a display device comprising sets of row and column address conductors, a row and column array of electro-optic display elements operable to produce a display, each of which is connected in series with a two terminal non-linear switching device between a row conductor and a column conductor, and a drive circuit connected to The sets of row and column address conductors for applying selection signals to the row address conductors to select the rows of display elements and data signals to The column address conductors to drive the selected display elements to produce a required display effect. The invention is concerned also with methods of driving such display devices.
The display device may be a liquid crystal display device used to display alpha-numeric or video information. The two terminal non-linear switching devices commonly used in such matrix display devices comprise thin film diode devices, MIMs, diode rings or back to back diodes which are bidirectional and largely symmetrical. The capacitive display elements in these devices are addressed by sequentially applying a selection voltage signal to each one of the set of row address conductors in turn in a respective row address period and applying in synchronised relationship data signals to the other set as appropriate to charge the display elements to a level providing the desired display condition, which following the row address period is subsequently held to maintain the display condition until They are again selected in a following field period. Conventionally, the data signals comprise amplitude modulated (analogue) voltage pulse signals of substantially identical and constant duration, related to the duration of the row address period, and whose amplitudes are varied to determine the display element voltage and produce the display effect required.
Display devices of the above kind and methods of driving such are described in U.S. Pat. No. 5,159,325 and GB-A-2129182. The method described in GB-A-2129182 entails the application to each row address conductor of a four level row drive waveform comprising a selection voltage level for a row selection interval of fixed duration followed by a second, hold, voltage level of less value but of the same polarity as the selection level which serves to hold the switching devices in the row off and which is maintained for at least a major portion of the time which elapses until the row conductor is next addressed so that the display elements are kept substantially at the level to which they were driven for that period. In successive field periods, the polarity of the selection and hold levels is inverted, thus making a four level signal waveform for each row conductor.
The method described in U.S. Pat. No. 5,159,325 employs a five level row scanning drive waveform which includes a reset voltage signal in addition to the selection signals and non-selection (hold) levels. The selection and hold levels are changed for successive fields and, together with the reset voltage signal, which may be regarded as an additional selection signal, form a five level signal waveform. Before presenting a selection signal, which together with the data signals provides the display elements of a row with a voltage of a certain sign, the display elements are charged through their non-linear switching devices to an auxiliary voltage level of the same sign and which lies at or beyond the range of voltage levels (Vth to Vsat) used for display purposes. This drive scheme helps to compensate for the effects of differences in the operating characteristics of the switching devices of the display device. Ideally, these devices should demonstrate substantially identical threshold and I-V characteristics so that the same drive voltages applied to any display element in the array produce substantially identical visual results. Differences in the thresholds, or turn-on points, of the non-linear switching devices can appear directly across the electro-optical material producing different display effects from display elements addressed with the same drive voltages.
Problems can arise if the operational characteristics of the switching devices drift over a period of time through ageing effects causing changes in the threshold levels. Because the voltage appearing across the electro-optic material depends on the on-current of the non-linear device, then if the on-current changes during the life of the display device the voltage across the electro-optic material also changes, which leads to inferior display quality and image storage problems. For switching devices such as thin film diode devices it has been found that this is ageing is due to current stressing effects. In EP-A-0699332 a modification to the form of the selection signals is proposed for reducing the extent of ageing effects. The form of the selection signals is tailored so that the peak current flowing through a switching device upon addressing, and thus the extent of ageing, is reduced. The difference in ageing between switching devices associated with display elements continually driven to different levels is also reduced. This is achieved by arranging that the selection voltage signals applied to the row conductors comprises a shaped voltage pulse signal whose magnitude increases gradually in a controlled fashion to a maximum amplitude during the row address period rather than the usual generally rectangular shape whose leading edge has a rapid and uncontrolled rise time which results in a high peak of current flowing through the device at the start of the selection address period. Through this shaping of the selection signals, the waveform of the current flowing through a switching device has a significantly reduced peak level.
In all these display devices the data signals applied to the display elements via the column conductors comprise amplitude modulated voltage signals whose level, together with the level of the selection signal, determines the voltage level of the display element, and thus its grey scale level, at the end of row address period.
Proposals have been made to drive an LC display device using two terminal non-linear switching devices by means of a pulse width modulation (PWM) drive scheme. This kind of drive scheme can offer attractions in certain types of display applications, particularly datagraphic, as purely digital, and hence for example lower power and less expensive, drive circuit ICs can be used. However, these proposals have generally proved unsatisfactory. GB-A-2186414 describes a PWM drive scheme but this involves a multiplex type drive technique rather than a true active matrix addressing technique. Unlike the above described row drive waveforms which include hold levels between successive selection signals that alternate in polarity in successive, positive and negative, fields the voltage present on the row conductors in the interval between selection signals is the same in both positive and negative fields. This means that the voltage on a display element capacitance decays away during the interval and the main contribution to the rms voltage across the LC display element is a voltage spike which occurs during the row address (selection) period only. The consequence of this is that the response speed of the LC material must be several field periods long in order to avoid flickering effects and this leads to a very slow response to changes in image content. Furthermore, the width (duration) of the selection signal is much more critical and a short selection signal duration can not be achieved without excessive drive voltage levels. In EP-A-0619572 a PWM drive scheme for a MIM LC display device is described in which a four level row drive waveform, having selection signals and hold levels that alternate in polarity in successive positive and negative fields, and similar to that described in GB-A-2129182, is used and in which the data signals determining grey-scale comprise pulse width modulated signals. However, it has been found that the range of grey-scales possible with the drive scheme described is severely limited so that the display device is not suitable for many display applications.
It is an object of the present invention to provide an active matrix display device using two terminal non-linear switching devices which can be driven using a PWM drive scheme and which is capable of displaying a wide range of grey-scales equivalent, for example, to that available for amplitude modulation of the data signals.
The present invention stems from a recognition that the shaping of the selection signals in the manner envisaged in EP-A-0699332 can be employed beneficially to allow the possibility of the display device being operated, and the display elements driven, in a manner which is different to that used by the display devices in the aforementioned publications and which can be advantageous for certain purposes, in addition to the reduction of ageing effects in the switching devices over a period of operation.
According to the present invention, there is provided an active matrix display device of the kind described in the opening paragraph, in which the data signals comprise pulse width modulated signals whose width determines a desired grey scale output from a display element, and in which the drive circuit is adapted to provide selection signals which comprise voltage pulse signals whose magnitude increases to a maximum voltage amplitude such that the current flowing through a non-linear switching device during the application of a selection signal tends towards a substantially constant value.
The invention is based on an appreciation of the reason for the problem of restrictions on the range of grey-scale possible in LC display devices using two-terminal non-linear switching devices and a pulse width modulation technique. When using conventional selection signals comprising substantially rectangular voltage pulses whose leading edge has a rapid rise time, it is found that the switching device turns on very quickly and that most of the charge supplied to the display element is transferred during an initial, short, part of the duration of the selection signal. Because of the extreme non-linearity between the pulse width of a data signal and the charge in this case, it is not possible to provide a wide range of grey-scales using such a drive scheme. If, however, the switching devices are controlled to give a more constant charging characteristic during a selection signal period then pulse width modulation can be used much more effectively and it readily becomes possible to drive the display elements to a wide range of grey-scales. This desired control of the switching devices is achieved by shaping the pulse signal constituting the selection signal in an appropriate manner. Through such shaping, then instead of most of the charge to (or from) a display element being passed through the switching device in an initial fraction of the duration of the selection signal, the flow of charge is regulated and is more constant over the duration of the selection signal rather than being peaked at the start. Such controlled display element charging (or discharging) rate is better suited for a PWM drive technique and allows a greater range of grey-scales than previously possible. The selection of the amount of charge supplied to the display element, and hence its voltage at the end of the row selection period, as determined by the width of the pulse width modulated data signal, becomes much easier by virtue of the nature of the resulting charge flow characteristic of the switching device.
In a preferred embodiment, the selection signals are shaped such that they initially increase rapidly to a predetermined level below the maximum and are then increased in a gradual and controlled manner, to the maximum amplitude. The gradual and controlled increase may be achieved by ramping smoothly and linearly or in staircase fashion. With such shaping the current flow through the switching device tends to become substantially constant, at a comparatively low level, throughout the selection period, thus enabling a substantially linear relationship between the pulse width and the charge to be realised. In other words, more constant charging during the selection period is achieved.
The selection signal, of a predetermined duration, defines a display element address period during which current can flow through the switching device to drive the display element and the PWM data signal controls the time for which this current actually flows so as to determine the display effect obtained. The data signal may determine the end of the interval within the address period when current flows through the switching device to drive the display element, in which case the beginning of said interval may be determined by the start of the selection signal. Alternatively, the data signal may be arranged to determine the start of the interval within the display element address period in which current flows through the switching device and the termination of this interval may be determined by the end of the selection signal. For this, an initial part of the data signal is preferably ramped in a linear manner to a predetermined voltage level so as to avoid possible current peaks which may occur if the voltage level on the column conductor is switched abruptly during the selection signal address period. With the known kind of row drive schemes in which positive and negative selection signals are applied to the row conductors in successive fields, a combination of these two approaches is preferably used such that in one field the start and end of the current flow interval are determined respectively by the start of the selection signal and the end of the data signal and in the next field the start and end of the interval are determined respectively by the start of the data signal and the end of the selection signal. This enables simplified column conductor signal waveforms to be used. In particular the number of polarity reversals needed in the column signal waveform is significantly reduced.